How can I adjust text size of the interface in Blender 2.8?
Open the preferences (Edit > Preferences). There are two options that allow you to change the font size. 

You can adjust the overall size of fonts and widgets using the Resolution Scale in the Interface tab. 



Alternatively if you want to change the font size without affecting the size of icons and widgets, you can modify the theme's font size in the Theme tab under Text Style. There you can adjust the size using the Points option for panel titles, widgets and widget labels separately.